In general, epoxy resins in the cured state have the following characteristics.    (1) Molded parts have less internal strain due to minimized shrinkage during molding and thus have high mechanical strength.    (2) The cured epoxy resins are fully electrically insulating.    (3) With respect to chemical resistance, the cured epoxy resins have good resistance to acids and alkalis though depending on the type of curing agent used.    (4) On use as adhesive or paint, the cured or modified epoxy resins have good bond strength and adhesion due to the presence of hydroxyl groups or ether bonds, and require no pressure for bonding.    (5) Appropriate modification allows epoxy resins to find a wider variety of applications.
By virtue of the superior physical, mechanical and electrical properties of cured products, prior art epoxy resin compositions are widely used as semiconductor encapsulating materials for protecting semiconductor devices such as diodes, ICs and LSIs from moisture and contaminants in the ambient atmosphere, for providing electrical insulation and for preventing mechanical failure; insulating materials for heavy-duty electric machines such as indoor insulators, instrument transformers, switch parts, power distribution transformers, outdoor insulated equipment, and rotating machines; laminates using epoxy varnish-based prepreg such as copper-clad laminates, epoxy resin FRP, filament windings, and voidless FRP; epoxy resin base adhesives and epoxy resin base paints.
On the other hand, the epoxy resin compositions in the cured state simultaneously have the drawback that they are very hard and brittle. Particularly when an interface exists between an epoxy resin composition and a different material, the difference in coefficient of expansion can cause internal stresses to generate in the cured epoxy resin composition whereby the cured resin becomes cracked. For instance, in the case of an epoxy resin encapsulant from which a plastic semiconductor package is molded, a thermal cycling test tends to invite cracks and delamination due to internal stresses at the interface between the resin and the metallic lead frame. In the case of epoxy resin base paint, the temperature difference from the environment can cause internal stresses to generate at the interface whereby the paint coating is cracked. To minimize the internal stresses that can cause such deficiencies, the epoxy resin composition in the cured state must be reduced in modulus of elasticity.
In the current situation where the operation of semiconductor devices becomes faster every year, prior art semiconductor epoxy resin encapsulants in which bisphenol or novolac type epoxy resins are cured with acid anhydrides or phenol novolac resins give rise to the problem that the package can heat up because the high dielectric constant prevents the encapsulant from following the frequency of operation of the semiconductor device. There is a need for low dielectric semiconductor epoxy resin encapsulants.